I have literally zero idea what are you're trying to express. There's no selection bias if everyone is FAANG is very well payed and everyone outside is not.
But if you really want to see something vaguely like that then go on blind and look for posts like "I'm making 650 as an E6 where should I go from here", the idea of which I'm sure just further confounds you but I'm sorry the data is very accurate (eg inside FB/Google/AMZN there's a group that openly discusses return offers for interns and it's always within a few percent of what is on the public side).
I don't see how there couldn't be sampling bias. Salary is a sensitive topic, and people who are making on the high range (within FAANG or outside of it) are going to be more likely to want to post about it and answer surveys about it than people in the low range. HN and Blind are obviously not random sampling. levels.fyi claims to use self-reported offer letters AND reach out directly to compensation folks at companies, which is better. But are these individual companies sharing unbiased data?
When one encounters average numbers that are a little higher than their reality, it's disappointing but believable. When one encounters average numbers that just look just crazy, one begins to ask "am I really this much of an outlier or should I suspect methodology/bias?"
Another thing about these threads is that the hundreds of people making the big bucks are always the ones happy to confirm these numbers, and the (hundreds? thousands? who knows?) making small bucks just silently lurk and think to themselves, "I'm doing something wrong." So you get selection bias. Then one person actually has the gall to post "I'm making less, clearly doing something wrong" and he gets tons of responses like "You're underpaid, brah, go get another job!" It's just not a helpful response.
I say this as one of the lurkers who is tired of pointing out that many of us don't make these so-called average salaries, because the response is always the same: "Dude, it's data--can't argue with that. Leetcode and/or try a different company" (as if we haven't done that already).